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The STM-556/RS-556 features an adjustable short-stroke gas piston system and a rotating bolt based on that of the AUG. In full-auto, it has a rate of fire between 600 and 800 rounds per minute. The five-position gas valve of the AUG has been moved to the front so that less hot gases are expended while shooting with a sound suppressor. Another unique feature of the STM-556 gas piston is that it can be extracted without tools and in no time at all—a very convenient feature that will make for quick cleaning and maintenance.

The barrel can be swapped out as quickly as an AUG’s, and the factory states that it can be performed in two seconds. I have never been so quick, but something like five seconds is easily attainable. And talking about changing out the barrel, let me add that in my experience the rifle holds its zero surprisingly well after removing and reinstalling the barrel. I consider 1 MOA to be a very stringent standard for an assault rifle, and the STM-556 I tested held this standard among barrels, which is very impressive.

Several barrels are available, and they’re all chrome lined with six-groove rifling in a 1-in-7- or 1-in-9-inch twist rate depending upon the intended application. Even though there are various standard barrel lengths—11.5, 14.5, 16, 18 and 20 inches—any special length the military customer might want can be easily manufactured. This is one of the beauties of the adjustable gas system; it will work no matter how the long or short the barrel is, unlike most direct-impingement AR-15s.

The monolithic upper receiver is CNC-machined from aircraft-grade aluminum bar stock, and it has steel inserts for enhanced durability and so the steel bolt can ride on steel for superior reliability. The bolt itself has a spring-loaded face with seven lugs as well as the AUG’s unique patented extractor with a semi-lug. As you can see, this isn’t a typical AR-15 bolt, and it’s much more reliable.

The STM-556/RS-556 features several standard and optional NATO accessory rails with receiver systems designed in accordance with MIL-STD-1913, STANAG 2324 and STANAG 4694. The top rail is a true STANAG 4694 design, which is more advanced, repeatable and stable than regular Picatinny rails, and it’s long enough to be fitted with Steyr’s superb AUG riflescope, an Aimpoint Micro with a 3X or even 6X magnifier, or any night-vision or thermal device.

The side rails are ready for laser designators or rangefinders, and the forend’s profile makes it easy to route control cables as needed. A 40mm grenade launcher can also be mounted on the bottom of the handguard. Moreover, the RS-556 is compatible with Rheinmetall’s modular “Future Soldier-Expanded System,” and can also be connected to other high-tech soldier systems.

The lower receiver of this Austrian marvel is basically a high-quality, billet-crafted, fully ambidextrous AR-15 lower. Heavily reinforced and redesigned in its weak points, it is compatible with STANAG magazines as well as standard AR accessories and triggers, which helps with maintenance and logistics. And talking about triggers, I found the trigger of the 2017 RS-556 version of the STM-556 way better than the one I tested back in 2012. It is more refined with a crisp single-stage release at just 3.5 pounds.

You might think that all these features come at a heavy price, especially with regard to weight. But with a 16-inch barrel installed and a fully loaded 30-round magazine, the Steyr weighs just over 9 pounds. Unloaded, it weighs 8.2 pounds. Finally, the collapsible, lightweight buttstock clicks into seven different length-of-pull positions, but the lower receiver can also use Magpul stocks or other aftermarket AR stocks.

You can change the barrel easily. Here you can see the Steyr AUG gas system in a marriage with an M4. Notice the black AUG in the background.

As demonstrated below, changing the barrel requires no tools and no disassembly and was done in front of us in less than a minute. Note the design of the barrel as it goes into locking position.

Credits to Steyr Mannlicher for bringing a real, used rifle to the table. It looks like this rifle has been shot. You can notice carbon build-up around the piston. Most of the competitors had their rifles behind glass, not to be touched.

The RS556 is already quite heavy. I think this will be quite a negative as Germany and other nations start to test the rifle.

Having written that, the handguard feels to short (one thing it shares with similar rifles from Heckler & Koch). Of course, this depends on how you hold your rifle when you shot, or for these types of rifles, how the Army you work for trains you.

Close-up of the gas position. I cannot tell if the handguard and barrel is free floating and I forgot to ask. If not it’s a major disadvantage for shooting barricades and with bipod or any other kind of support.